1. Field of the Invention
The present invention relates to wireless power feeding and, more particularly, to power control thereof.
2. Description of Related Art
A wireless power feeding technique of feeding power without a power cord is now attracting attention. The current wireless power feeding technique is roughly divided into three: (A) type utilizing electromagnetic induction (for short range); (B) type utilizing radio wave (for long range); and (C) type utilizing resonance phenomenon of magnetic field (for intermediate range).
The type (A) utilizing electromagnetic induction has generally been employed in familiar home appliances such as an electric shaver; however, it can be effective only in a short range of several centimeters because power transmission efficiency abruptly reduces when the wireless transmission distance is increased. The type (B) utilizing radio wave is available in a long range; however, it has small electric power. The type (C) utilizing magnetic field resonance phenomenon is a comparatively new technique and is of particular interest because of its high power transmission efficiency even in an intermediate range of about several meters. For example, a plan is being studied in which a receiving coil is buried in a lower portion of an EV (Electric Vehicle) so as to feed power from a feeding coil in the ground in a non-contact manner. The wireless configuration allows a completely insulated system to be achieved, which is especially effective for power feeding in the rain. Hereinafter, the type (C) is referred to as “magnetic field resonance type”.
The magnetic field resonance type is based on a theory published by Massachusetts Institute of Technology in 2006 (refer to Patent Document 1). In the magnetic resonance type, a resonance circuit (LC circuit) is formed on both the power feeding side and power receiving side, respectively. The resonance frequency of the power feeding side resonance circuit and that of the power receiving side resonance circuit are made to coincide with each other. When the power feeding side resonance circuit is made to resonate at a resonance frequency fr1, the power receiving side resonance circuit resonates at a resonance frequency fr1. At this time, AC power can be fed with the maximum power transmission efficiency (refer to Patent Document 6).